Linux Virtualization - Apr 2023 - [PATCH vhost v6 08/11] virtio_ring: introduce virtqueue_dma_dev()

On Mon, Apr 10, 2023 at 01:14:13PM +0800, Jason Wang
wrote:
> > But rethink, this is unreliable. Some platforms have returned their
own ops,
> > including X86.
> >
> > Because the priority of dev->dma_ops is higher than
get_arch_dma_ops(), we
> > should not let dev->dma_ops be null. We should define a set of
dma_ops to
> > actively implement direct dMA.
> 
> Then this will duplicate with existing DMA API helpers. Could we tweak
> the sparc or introduce a new flag for the device structure then the
> DMA API knows it needs to use direct mapping?
> 
> Adding Christoph for more comments.
Code outside of the core kernel/dma/ code has no business doing
anything with the dma_ops.  WTF is going on?

On Mon, 10 Apr 2023 08:27:14 -0700, Christoph Hellwig <hch at
infradead.org> wrote:
> On Mon, Apr 10, 2023 at 01:14:13PM +0800, Jason Wang wrote:
> > > But rethink, this is unreliable. Some platforms have returned
their own ops,
> > > including X86.
> > >
> > > Because the priority of dev->dma_ops is higher than
get_arch_dma_ops(), we
> > > should not let dev->dma_ops be null. We should define a set of
dma_ops to
> > > actively implement direct dMA.
> >
> > Then this will duplicate with existing DMA API helpers. Could we tweak
> > the sparc or introduce a new flag for the device structure then the
> > DMA API knows it needs to use direct mapping?
> >
> > Adding Christoph for more comments.
>
> Code outside of the core kernel/dma/ code has no business doing
> anything with the dma_ops.
Do you mean we should not change the dma_ops from the outside of the core
kernel/dma/?

How about adding one new "dma_direct" to struct devive?

--- a/include/linux/dma-map-ops.h
+++ b/include/linux/dma-map-ops.h
@@ -88,6 +88,9 @@ struct dma_map_ops {

 static inline const struct dma_map_ops *get_dma_ops(struct device *dev)
 {
+       if (dev->dma_direct)
+               return NULL;
+
        if (dev->dma_ops)
                return dev->dma_ops;
        return get_arch_dma_ops();


Thanks.


> WTF is going on?